1. Field of the Invention
The present invention relates to ferroelectric liquid crystal devices, and more particularly to a ferroelectric liquid crystal device having substrates, voltage application means, an orientation control layer and a ferroelectric liquid crystal layer which comprises a specific ferroelectric liquid crystal composition.
2. Description of the Prior Art
Liquid crystal display devices most widely used presently are those utilizing a nematic phase but having the drawback that they are not adapted for large-capacity display, for example, with 1000.times.1000 lines. For instance, usual twisted nametic (TN) liquid crystal display devices decrease in contrast with an increase in the number of lines, so that it is practically impossible to fabricate contrasty large-capacity liquid crystal devices of this type with 1000.times.1000 lines. To overcome the drawback of TN liquid crystal display devices, supertwisted nematic (STN) liquid crystal display devices and double supertwisted namatic (DSTN) liquid crystal display devices have been developed, whereas these devices still have the drawback of decreasing in contrast and in the speed of response with increasing number of lines. Such devices presently available are therefore limited to a display capacity of about 1000.times.720 lines. On the other hand, the prior art has already provided liquid crystal display devices of the active matrix type wherein thin-film transistors (TFT) are arranged on a substrate. Although it is technically possible to give devices of this type a large display capacity, for example, of 1000.times.1000 lines, these devices have the drawback of necessitating a long production process and being low in yield and therefore very costly to fabricate.
As promising means for overcoming the foregoing problems, ferroelectric liquid crystal display devices are proposed which operate on a different principle from the TN display device (see N. A. Clark et al., Appl. Phys. Lett., 36, 899(1980)). The proposed device utilizes the chiral smectic C phase, chiral smectic I phase and the like of ferroelectric liquid crystals. The device can be given a great display capacity with an improved speed of response since the memory property of the crystals is utilized. Furthermore, the device can be produced at a low cost since there is no need to use active components such as thin-film transistors. The ferroelectric liquid crystal display device also has the advantage of being wide in field of view. Thus, the device appears very promising as a large-capacity display device having at least 1000.times.1000 lines.
The liquid crystal material for use in the ferroelectric liquid crystal display device wherein smectic C phase is utilized must of course exhibit the smectic C phase over a wide temperature range around room temperature and needs to fulfill various other requirements. First, the device for large-capacity display must have high-speed responsitiveness, and from this viewpoint, the liquid crystal material needs to be highly amendable to spontaneous polarization and low in viscosity. Further the material needs to exhibit satisfactory orientation and bistability when used for the liquid crystal cell. It is also desired that the material be great in tilt angle which is relevant to the contrast and brightness of liquid crystal display.
At present, however, it is impossible for a single compound to fulfill all the desired requirements, so that a plurality of compounds are usually mixed together for use as a liquid crystal composition. To prepare a liquid crystal composition filfilling the requirements for actual use, it is necessary to use numerous single liquid crystal compounds having a wide variety of properties. It is sometimes likely that compounds which per se exhibit no liquid crystal properties will be useful as components of the liquid crystal composition.